bug#34260: 27.0.50; Emacs randomly hangs during redisplay

[John Shahid]

I have updated to the latest master a while ago and since then my Emacs
have been acting weird and becomes unusable.  I tried to turn on
debug-on-quit but I Emacs is always inside redisplay when I hit C-g.
Almost always the stacktrace has somewhere a call to `message` which in
turn calls `redisplay` and it just hangs and consumes 100% of the cpu.
I tried to debug the proces using gdb and found out that Emacs is always
inside `redisplay_internal' when I stop it using `kill -TSTP emacs'.

Worth noting that this problem isn't predictable and is hard to
reproduce.  It also might be related to EXWM and commit `e567ac1495'.

> 436c225f1b * Unbreak the macOS NS build
> 957090b795 *   Merge from origin/emacs-26
>            |\
> c9f6f86814 | * Prevent redrawing if frame is garbaged
> 00ba2267ed | * Attempt to fix hangs on MS-Windows due to C-g
> e567ac1495 * | Run window change functions during redisplay

I reverted my local copy of Emacs to '470082de55: List lengths are
always fixnums now' and haven't ran into any issues since then.

Let me know how I can debug this issue further.  Would the display
engine trace help in this case ?  Emacs doesn't enter an infinite
recursion but from the CPU usage it is entering some sort of infinite
loop, but I can't figure out why.


In GNU Emacs 27.0.50 (build 4, x86_64-pc-linux-gnu, GTK+ Version 3.24.4)
 of 2019-01-30 built on amun
Repository revision: 1d804268c4656e9a7561a0bdff728d932ffd3812
Repository branch: wtf
Windowing system distributor 'The X.Org Foundation', version 11.0.12003000
System Description: Arch Linux

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